The implementation and use of high voltage output systems within ICDs is well known. Generally, ICDs have high voltage (HV) output capacitors, typically valve metal electrolytic capacitors, which are typically charged to a substantially full (or maximum) preprogrammed charge via high current battery systems, such as silver vanadium oxide (SVO) battery cells, coupled to DC-to-DC voltage converters in order to generate cardioversion/defibrillation (C/D) shocks. An example of the high voltage charging system for an existing ICD is described in U.S. Pat. No. 5,372,605, for example. The HV output capacitors are charged up to the programmed voltage when tachyarrhythmia detection criteria are met and a C/D shock is to be delivered by discharging the HV output capacitors through the heart between C/D electrodes.
The term “valve metal” stands for a group of metals including aluminum, tantalum, niobium, titanium, zirconium, etc., all of which form adherent, electrically insulating, metal oxide dielectric films or layers upon anodic polarization in electrically conductive solutions. Wet electrolytic capacitors essentially consist of an anode electrode, a cathode electrode, a barrier or separator layer for separating the anode and cathode, and an electrolyte. In cylindrical electrolytic capacitors, the anode electrode is typically composed of wound anodized aluminum foil in which subsequent windings are separated by at least one separator layer. The anodes in a flat electrolytic capacitor (FEC) may consist of stacked sheets of aluminum that are electrically connected together. In another type of capacitor a valve metal powder is pressed, sintered and formed into a typically unitary anode electrode, and the anode is separated from at least one cathode by a electrically insulative separator layer as is known in the art and as described further below. For an FEC, typically a plurality of aluminum sheets are etched or perforated to increase surface area. For both FEC- and pressed and sintered-type capacitors, an oxide dielectric is formed upon on exposed surfaces of the anode (the pressed and sintered structure or etched or the perforated sheets) when the anode is immersed in a formation electrolyte while electrical current circulates therethrough during manufacture. Examples of electrolytic capacitors are disclosed, for example in commonly assigned U.S. Pat. No. 6,006,133 and in U.S. Pat. Nos. 6,249,423, 6,283,985, and 5,926,362.
In order to conserve ICD battery power, the HV output capacitors remain in an uncharged state when not in use. However, the metal oxide dielectric tends to degrade when the HV output capacitors are left in an uncharged state between charging to deliver C/D shocks. When it becomes necessary to charge the HV output capacitors, there can be a considerable leakage current occurring between the anode and cathode electrodes of the HV output capacitors. This leakage current can prolong the time that it takes to charge the HV output capacitors to the desired C/D voltage, and the delay can possibly delay necessary electrical therapy delivery to a patient. Moreover, this leakage current also requires that more battery energy be expended to charge the HV output capacitors to the desired C/D voltage. Consequently, the leakage current can further result in excessive consumption of limited battery power thereby decreasing the longevity of the ICD.
Thus, although such valve metal electrolytic capacitors have a relatively high energy density per volume, such capacitors tend to degrade electrochemically over time thereby increasing the charge time required to fully charge the HV output capacitor system. Similarly, the SVO battery cells also have a tendency to degrade electrochemically over time if they are not discharged due to the increased equivalent series resistance (ESR) within the battery that decreases the current output capabilities of the battery.
The conventional solution to both of these problems has been to conduct a periodic reforming of the high voltage output system of an ICD by rapidly charging the HV output capacitor system to its full rated voltage and then allowing discharge through a non-therapeutic load (e.g., discharge through a resistive load) or allowing discharge via leakage current(s). In this way, both the high current battery system and the HV output capacitor system are exercised so as to reform the electro-chemistries of each system, thereby reducing the impact on charge performance and component life due to electrochemical degradation over time. Originally, this reforming process was accomplished manually by having a patient visit the physician every two to three months, at that time the physician would fully charge the capacitor(s), but not deliver, a full voltage rated C/D therapy shock. Presently, the reforming of the high voltage output system is accomplished automatically by the ICD based on a fixed time period (e.g., every month, every six months), at the end of that a full charge cycle of the HV output capacitor system is automatically conducted. The physician can program the fixed time.
For example, for a typical HV output capacitor used in an ICD, the HV output capacitor will be charged during reforming maintenance to approximately 800 volts that requires the battery to provide approximately 55 joules of energy. This is a considerable expenditure of battery energy, which significantly reduces the longevity of the battery. Moreover, the prior art systems that periodically charge the HV output capacitors often end up charging the HV output capacitors when dielectric has not degraded to the point where the leakage current that would occur during the generation of a therapeutic waveform would present a problem. Consequently, while periodically reforming the HV output capacitor during periods of non-use to the HV output capacitor's peak voltage may reduce the leakage current during therapeutic waveform generation, the reduction in leakage current is accomplished at a significant cost in terms of battery and device longevity.
While this kind of simple periodic reform cycle was more than effective for early ICDs where the life span of the device was typically less than three years and the battery budget could easily support the periodic reform cycles, newer ICDs are smaller and have much longer life spans. An example of such an ICD that is used as a prophylactic device is described in U.S. Pat. No. 5,439,482. In these newer designs for an ICD, battery power is at more of a premium than in previous designs and the periodic reforming of the high voltage output system can represent a significant portion of the battery budget over the life of the device.
Alternate techniques for accomplishing reforming of the battery system and the HV output capacitor system are disclosed in U.S. Pat. Nos. 5,861,106, 5,899,923 and 5,690,685. In the '923 patent, a system is disclosed for measuring the leakage current of the HV output capacitor system at a relatively low voltage and using this value to estimate whether the HV output capacitor system needs to be reformed. By utilizing a low voltage test process, battery power is conserved and full voltage reforming is conducted only when it is determined that the HV output capacitor is in need of reforming. In the '685 patent, a technique is disclosed for measuring an electrical parameter of the battery system and using this value to determine whether the battery system needs to be reformed. Again, battery power is conserved by only performing a full voltage reform when it is determined that the internal resistance of the battery system has increased to the point where charge performance is degraded. A system for selectively reforming the high voltage output systems of an ICD based on the charge history and charge performance of the battery and capacitor systems so as to maintain charge performance while conserving battery power is disclosed in the above-referenced '106 patent.
While such approaches may offer promise, they suffer from the disadvantage of potentially requiring additional circuit within the ICD to implement. Therefore, it would be advantageous to develop a more efficient system and algorithm for reforming the oxide layers of the HV output capacitors and battery of an ICD. It would be advantageous to develop a simpler capacitor oxide layer reform system and algorithm or process that does not require significant additional circuitry within the ICD.
The rapid charging of the ICD capacitors to the full output voltage or a lesser reforming voltage to reform the oxide layers of the capacitor plates can result in very high local current densities that may result in localized oxide layer defects and residual stresses that can allow the capacitor to degrade further and be less efficient during subsequent shock therapy delivery or reform charge and discharge cycles. Moreover, the rapid charging of the HV output capacitors during the charge phase of the reform charge and discharge cycle increases resistive power losses within the battery, thereby decreasing device longevity. Therefore, it would also be advantageous to develop a capacitor oxide layer reform system and algorithm or process that reduces the extent of such oxide layer damage and resistive power losses.